1. Field of the Invention
The present invention relates generally to an ion implantation system. Specifically, the present invention relates to an ion implantation system to minimize metal contamination of a semiconductor wafer prepared thereby.
2. Description of the Background Art
Generally, an ion implantation system is composed of an ion source, an ion drawing out region, a mass analyzing region, an acceleration region, and a scanning(convergence/deflection) region. A vacuum vessel defines a path of ion beam drawn out from the ion source to an end station where a silicon wafer to be deposited is installed. A Faraday gauge and an acceleration electrode are installed downstream of the scanning region. Plurality of apertures are installed at plurality of portions of the system. In the vacuum vessel, a beamline and a chamber of the mass analyzing region, which occupies most of the vessel, are formed of stainless steel materials, such as Fe, Cr and Ni. The Faraday gauge and the acceleration electrode are formed of aluminum (Al). The apertures, the scanning region and a lens electrode are formed of graphite. When the silicon wafer is ion implanted using such ion implantation system, inner wall of the beamline through which the beam passes are sputtered by ions. Therefore, metals, e.g., Fe, Cr and Al, are brought into the silicon wafer.
FIGS. 1 and 2 are graphs showing a relationship between the depth of the wafer and ion concentration when arsenic (As) ions were implanted into the silicon wafer under the conditions of 70 keV, 1e17 using the aforementioned ion implantation system. Measurement is done by secondary ion mass spectrometry (SIMS). FIG. 1 shows contamination level of the wafer by Cr and Al, on the other hand, FIG. 2 shows that by Fe. As sown in the figures, 14 ppm of chromium (Cr), 4.9 ppm of Al, and 27 ppm of iron (Fe) is respectively contaminated in the wafer against the amount of implanted As ions. Such levels of metal contamination in the wafer increases leakage current of MOS transistor formed on the wafer.
Alternatively, an ion implantation system including a plurality of protection boards (apertures) to protect passing of metals sputtered is disclosed in Extended Abstracts of the 1991 International Conference of Solid State Devices and Materials, Yokohama, 1991, pp. 565-567.
Referring to FIG. 8 showing such ion implantation system, a plurality of protection boards 1 and 2 formed of silicon are installed downstream of a scanning region 3 of the Y direction and downstream of a scanning region 4 of the X direction to protect passing sputtered metals. Numeral 5 designates a silicon wafer. When the ion beam is directed toward the wafer with diverging ions (repulsing of ions from each other), the ion beam sputters the wall of the beamline upstream of the protection boards 1 and 2. However, most particles of metals, such as Fe and Cr, generated by sputtering are protected by the protection boards 1 and 2. Therefore, very few metal particles can reach the wafer because the ion beam travels a long distance along the beamline toward the wall while the width of the ion beam is relatively narrow.
Additionally, it has been proposed that materials used in the construction of the system be mostly replaced with silicon parts. Though this is very effective to perfectly eliminate metal contamination, the cost for manufacturing becomes great. Additionally, durability of the system is deteriorated if all parts are replaced with silicon parts.